1. Field of the Invention
This invention relates in general to an electrophotographic or elestrostatic-recording image forming apparatus for forming an image on a recording material and subsequently fixing the image on the recording material by a fixing means. In particular, the present invention may be suitably applied to a color electrophotographic apparatus or the like.
2. Description of the Related Art
In a conventional image forming apparatus, such as a color electrophotographic apparatus or the like, a technique is known wherein toner images of a plurality of colors formed on a photosensitive drum, serving as an image bearing member, are individually transferred onto a recording material, such as paper or the like, and carried on a transfer drum, serving as a recording material carrying means, with superposing respective images. After the image transfer, the recording material is separated from the transfer drum, and is conveyed to a fixing device by a conveying belt.
The superposed toner images of the plurality of colors formed on the recording material are fused by means of heat and pressure in the fixing device comprising a fixing roller and a pressing roller to provide a full-color image, and the image is fixed on the recording material.
The above-described image forming apparatus, however, has the following problems.
That is, normal paper has generally been used as the recording material. Particularly, paper having a weight of about 50-90 g/m.sup.2 has been mostly used. However, in accordance with recent various needs, more kinds of recording materials have been desired. In some cases, thick paper having, for example, a weight of 90-150 g/m.sup.2, a recording material made of a resin-like film, or the like, has been used. Particularly, the resin-like recording material has been used in most cases as a transparent OHP (overhead projector) film. When using such recording materials, in fixing an image, it is necessary to perform a fixing operation at a speed lower than in conventional cases.
For example, in the present case, if the process speed of the main body is assumed to be 84 mm/sec, the fixing speed (the recording material conveying speed in the fixing device) is usually 84 mm/sec when normal paper is used as the recording material.
However, when a fixing operation is performed for thick paper used as the recording material, the above-described fixing speed is insufficient. Since the heat capacity for fusing toner particles is insufficient, the gloss of the image disappears after the fixing operation, or so-called "cold offset", wherein toner particles do not adhere to the recording material, occurs. Cold offset apts to occur particularly in a color image forming apparatus. Cold offset occurs because toner particles adhere onto the recording material and form thick multiple layers, i.e., 2-4 layers, and the heat from the fixing roller side is not effectively transmitted to the lowermost layer. Therefore, the toner particles in the lowest layer are not effectively fused. Furthermore, since the heat from the back surface of thick paper does not effectively reach toner particles in the lowermost layer, toner particles in the lowermost layer are not effectively fused. As a result, the anchoring effect of the toner for the paper cannot be obtained, and so the toner peels from the paper. Accordingly, when performing a fixing operation using thick paper as the recording material, it is necessary to perform the fixing operation at a speed lower than when performing a fixing operation using normal paper as the recording material. The fixing speed in such a case is set to 25 mm/sec.
When using a resin-like film as the recording material, since heat is not effectively transmitted as in the case of thick paper, and the surface of the film is very smooth, the anchoring effect of the toner cannot be obtained, and so the toner peels off. Hence, it is necessary to perform a fixing operation at a low speed (25 mm/sec) as in the case of thick paper.
When using an OHP film as the recording material, in order to obtain a color-forming property and a light-transmitting property of an image, and to provide excellent colors of color toners when projecting the image, it is necessary to fix toner particles at a low speed (a fixing speed of 25 mm/sec) in order to fuse the toner particles, mix colors of the toner particles, and sufficiently fix the toner particles.
In the above-described image forming apparatus, a technique has been known wherein a plurality of sheets of the recording material are simultaneously carried at different positions on the transfer drum in order to increase the copying speed. For example, a case will be considered wherein the process speed is 84 mm/sec (the recording-material conveying speed of the transfer drum is also 84 mm/sec) as described above. A transfer operation is performed while carrying two sheets of the recording material on the transfer drum, the two sheets are separated from the transfer drum, and the two sheets of the recording material conveyed by the conveying belt successively pass through the fixing device. When the fixing device performs a fixing operation at a low speed (25 mm/sec), the second sheet of the recording material overtakes the first sheet being fixed. As a result, images on the first and second sheets of the recording material may be disturbed, or jamming of the sheets may occur.